1. Field of the Invention
The present invention relates to a porous silicon-based electrode active material and a secondary battery comprising the same.
2. Description of the Prior Art
Since the discovery of electricity in the 1800s, primary batteries have developed into secondary batteries, and batteries having low operating voltage have developed into batteries having high operating voltage. Among this variety of batteries, lithium secondary batteries are leading 21st battery technology and are receiving attention as energy storage systems for a variety of applications, including mobile phones and electric vehicles.
Lithium secondary batteries are energy storage devices in which lithium ions move from the anode (negative electrode) to the cathode (positive electrode) during discharge and move from the cathode to the anode during charging when storing energy in the batteries. The lithium secondary batteries have high energy density and low self-discharge rate compared to other types of batteries, and thus are used in a wide range of applications.
General lithium secondary batteries comprise a cathode, an anode, an electrolyte and a separator. In early lithium secondary batteries, lithium metal was used as the anode active material, but was replaced with carbon-based materials such as graphite, because of safety concerns resulting from the repeated charge/discharge cycles. The potential of the electrochemical reaction of the carbon-based anode active material with lithium ions is similar to that of lithium metal, and the change in the crystal structure thereof during the intercalation/deintercalation of lithium ions is low. Thus, the carbon-based anode active material can be repeatedly charged and discharged and has excellent charge/discharge cycle characteristics.
However, in recent years, as the lithium secondary battery market has expanded from small-sized lithium secondary batteries for mobile devices to large-sized lithium secondary batteries for automobiles, there is a newfound need for a technology that can achieve the high capacity and high output of anode active materials. Thus, non-carbon-based anode active materials, including silicon, tin, germanium, zinc and lead-based materials, have been actively developed, which theoretically have capacities higher than carbon-based anode active materials.
Among these, silicon-based anode active materials have a capacity of 4190 mAh/g, which is 11 times higher than the theoretical capacity (372 mAh/g) of the carbon-based anode active materials, and thus have received attention as a substitute for the carbon-based anode active materials. However, in the case of using silicon alone as the anode active material, its volume expands by a factor of 3 or more when it is intercalated by lithium ions. For this reason, the battery capacity decreases as the number of charge/discharge cycles increases, and safety concerns also arise. Thus, in order to commercially use silicon as an anode active material, many studies are required into that battery.
As a result, studies on silicon-based composites have been actively conducted. Among these, studies have been made into the use of a silicon-based material in combination with a carbon-based material. This method was developed to minimize the volume expansion of the silicon active material in order to increase capacity and charge/discharge cycle characteristics. The most fundamental method for synthesizing the composite is to coat the silicon-based material with carbon. This improves the electrical conductivity between active material particles and the electrochemical properties and the properties of the electrochemical reaction with an electrolyte and reduces the volume expansion of the silicon-based particles, resulting in an increase in the battery lifetime. However, there is a problem in that the initial charge/discharge efficiency is deteriorated due to the formation of an irreversible phase by the silicon-based material during initial charge/discharge cycling.